12.15 Casts of aluminum were subjected to four standard heat treatments and their tensile strengths were measured. Five measurements were taken on each treatment, with the results (in 1,000 psi), shown below.

Perform an analysis of variance. Do the mean tensile strengths differ from treatment to treatment, at the 5% significance level?

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Treatment A B C D 35 41 42 31 31 40 49 32 40 43 45 30 36 39 47 32 32 45 48 34